Phase transitions and precrystallization processes in a water–protein–electrolyte system

Abstract

To study the conditions under which supramolecular dynamic aggregates of proteins (clusters) are formed in protein solutions, the thermodynamic stability of the model system water–biopolymer–electrolyte was analyzed. The analytical expression of the coefficient of the stability of the system ( ∂ μ 1 / ∂ m 2 ) m 3 was obtained, where μ 1 is a chemical potential of water and m 2 and m 3 are molar concentrations of the biopolymer and the electrolyte. The expression was used to obtain equations of spinodal and two quasi-spinodals that converge at one point in phase diagram. They correspond to the lines of critical and supercritical phase transitions (PTs) that give rise to the various structural–dynamic states of solution. These equations relate the critical composition of the system ( m 2/ m 3) cr to the specific characteristics of the biopolymer and electrolyte ( m 2/ m 3) cr= f(ν, z, Δ), where ν is the number of the electrolyte ions that interact with the biopolymer, z is its charge, and Δ is the function of concentration-dependent variation in the coefficient of activity of the electrolyte. In the region of one of the quasi-spinodals, in a narrow electrolyte concentration range, the most favorable conditions for the formation of crystalline-phase nuclei have been created. The author presents experimental data obtained using the EPR spin-label method that provide evidence for PTs in solutions of serum albumin molecules which are in a qualitative agreements with phase diagram predictions. The mechanism of protein cluster formation is discussed.